Despite a great deal of study, we still know surprisingly little about the factors responsible for the virulence of M. tuberculosis, either in the mouse and in humans. Studies carried out using aerogenically challenged mice indicate that the virulent strain H37Rv continues to grow within the lung even after clear evidence of an effective immune response has been expressed within the liver and spleen. On the other hand, the avirulent variant, H37Ra produces little or no initial growth within the lung following aerogenic infection and there is a relatively rapid clearance of the organisms from both the lungs and the spleen. These mice develop little or no acquired resistance to a subsequent virulent challenge with H37Rv. A number of anxotrophic mutants of Mtb H37Rv as well as H37Ra recombinant bearing randomly selected DNA fragments from M. tuberculosis Erdman Bcgr have been selected by infecting normal mice with mixtures of recombinants and following the growth of infecting organisms in the lungs of C57BL/6 (Bcgs) and A/I (Bcgr) mice and guinea pigs, comparing the resulting growth curves with those for M. tuberculosis H37Rv and H37Ra as positive and negative growth controls. Recombinants (hygromycin-resistant) recovered from the lungs after 3 months (at which time the H37Ra population is no longer detectable) will be tested for the presence of the ivg gene and for their ability to grow within the mouse lung. Logarithmically growing cultures of selected recombinants will be examined for the presence of specific protein sensitins able to induce DTH responses in the footpads of the infected mice. Mice will also be vaccinated with the selected recombinants and the resulting level of acquired resistance to an aerogenic challenge will be compared with that seen in BCG and H37Ra vaccinated controls.